The invention relates to an advance arrangement for use in controlling the timing of fuel delivery by a high pressure fuel pump intended for use in a compression ignition internal combustion engine.
In a conventional rotary fuel pump, the angular position of a cam ring is adjusted by means of a servo-advance arrangement to control the timing of fuel delivery by the pump. The advance arrangement includes an advance piston which is slidable within a bore and which cooperates, in use, with a cam arrangement of the fuel pump to adjust the timing of fuel delivery by the pump. A servo-piston is slidable within a further bore provided in the advance piston and is movable in response to fuel pressure variations within a servo control chamber, the pressure of fuel delivered to the servo control chamber being dependent upon engine speed. If the engine speed increases, fuel pressure delivered to the servo control chamber (transfer pressure) is increased and the servo piston is moved to increase the pressure of fuel applied to the advance piston, thereby causing the advance piston to move to advance the timing of fuel delivery by the pump. If engine speed is reduced, the pressure of fuel delivered to the servo control chamber is reduced causing the servo piston to move to reduce fuel pressure acting on the advance piston, as a result of which timing of fuel delivery is retarded.
It is also known to provide a light load advance arrangement including a light load sensing piston which is movable relative to the advance piston against the action of a light load control spring. A force due to fuel pressure within the light load control chamber acts on the light load piston, in combination with the light load control spring, to determine the relative axial positions of the light load piston and the advance piston and, hence, the maximum permitted level of advance. A control valve is operable to control the pressure of fuel within the light load control chamber depending on the load under which the engine is operating. Thus, depending on the engine load, the pressure of fuel acting on the light load piston varies and the position of the light load piston changes. The movement of the light load piston results in movement of the servo-piston which, in turn, results in movement of the advance piston, thereby causing movement of the cam ring to adjust the timing of fuel delivery by the pump.
It is also known to provide the pump with a cold advance arrangement to permit adjustment of fuel delivery timing depending on engine temperature. The cold advance arrangement includes a temperature control valve arranged to increase fuel pressure within the light load control chamber if the temperature of the engine falls below of predetermined amount. Increased pressure within the light load control chamber results in movement of the light load piston and therefore adjusts the relationship between the position of the advance piston and the temperature of the engine.
For some engines to start and operate properly in cold conditions, it is necessary to advance injection timing to accommodate longer combustion delays. However, it is only possible to adjust the degree of cold advance if transfer pressure is sufficiently high, otherwise the force acting to move of the advance piston to advance timing will be insufficient to overcome the force due to the light load control spring. In conventional pumps, it is only possible to apply cold advance if the engine speed is between idling and rated speed. When conventional pumps of the aforementioned type are used in certain engine applications it is not therefore possible to compensate for cold engine conditions upon engine start up.
It is an object of the present invention to overcome the aforementioned problem.